Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems, and more particularly to mechanisms and techniques, for performing a marine seismic survey using water-coupled underwater nodes that carry appropriate seismic sensors.
Discussion of the Background
Marine seismic data acquisition and processing generate a profile (image) of a geophysical structure under the seafloor. While this profile does not provide an accurate location of oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of these reservoirs. Thus, providing a high-resolution image of the geophysical structures under the seafloor is an ongoing process.
Reflection seismology is a method of geophysical exploration for determining the properties of earth's subsurface, which is especially helpful in the oil and gas industry. Marine reflection seismology is based on using a controlled source of energy that sends the energy into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
A traditional marine system for generating seismic waves and recording their reflections off the geological structures present in the subsurface is illustrated in FIG. 1. A vessel 10 tows an array of seismic receivers 11 provided on streamers 12. The streamers may be disposed along any depth profile relative to a surface 14 of the ocean. The streamers may be disposed to have other than horizontal spatial arrangements. The vessel 10 also tows a seismic source array 16 that is configured to generate a seismic wave 18. The seismic wave 18 propagates downwards toward the seafloor 20 and penetrates the seafloor until eventually a reflecting structure 22 (reflector) reflects the seismic wave. The reflected seismic wave 24 propagates upward until it is detected by the receiver 11 on the streamer 12. Based on the further analyses of the data collected by the receiver 11, an image of the subsurface is generated. The seismic source array 16 includes plural individual source elements.
FIG. 2 shows a vessel 40 towing two cables 42 provided at respective ends with deflectors 44. Plural lead-in cables 46 are connected to streamers 50. The plural lead-in cables 46 also connect to the vessel 40. The streamers 50 are maintained at desired distances from each other by separation ropes 48. Plural individual source elements 52 are also connected to the vessel 40 and to the lead-in cables 46 via ropes 54.
However, this traditional configuration is expensive as the cost of the streamers is high. In addition, this configuration might not provide accurate results as the water surface noise may interfere with the recordings. To overcome the latter problem, new technologies deploy plural seismic sensors on the bottom of the ocean to provide a coupling between the sensors and the ocean floor.
One such new technology is incorporated into the ocean bottom station (OBS) nodes. OBS are capable of providing better data than conventional acquisition systems because of their wide-azimuth geometry. Wide-azimuth coverage is helpful for imaging beneath complex overburdens like those associated with salt bodies. Salt bodies act like huge lenses, distorting seismic waves that propagate through them. To image subsalt targets, it is preferable to have the capability to image through complex overburdens, but even the best imaging technology alone is not enough. A good illumination of the targets is necessary. Conventional streamer surveys are operated with a single seismic vessel and have a narrow azimuthal coverage. If either the source or the receiver is located above an overburden anomaly, the illumination of some targets is likely to be poor. OBS nodes can achieve wide-azimuth geometry.
Additionally, OBS nodes are much more practical in the presence of obstacles such as production facilities. For the purpose of seismic monitoring with repeat surveys (4D), OBS have better positioning repeatability than streamers. Furthermore, OBS nodes provide multi-component data. Such data can be used for separating up- and down-going waves at the seabed, which is useful for multiple attenuations and for imaging using the numerous pieces of data. In addition, multi-component data allow for the recording of shear waves, which provide additional information about lithology and fractures and sometimes allow to image targets that have low reflectivity or are under gas clouds.
U.S. Pat. No. 6,932,185, the entire content of which is incorporated herein by reference, discloses this kind of node. In this case, the seismic sensors 60 are attached to a heavy pedestal 62, as shown in FIG. 3 (which corresponds to FIG. 4 of the patent). A station 64 that includes the sensors 60 is launched from a vessel and arrives due to its gravity, to a desired position. The station 64 remains on the bottom of the ocean permanently. Data recorded by the sensors 60 is transferred through a cable 66 to a mobile station 68. When necessary, the mobile station 68 may be brought to the surface to retrieve the data.
Although this method provides a better coupling between the seabed and the sensors, the method is still expensive and not flexible as the stations and corresponding sensors are left on the seabed.
An improvement to this method is described in European Patent No. EP 1 217 390, the entire content of which is incorporated herein by reference. In this document, a sensor 70 (see FIG. 4) is removably attached to a pedestal 72 together with a memory device 74. After recording the seismic waves, the sensor 70 together with the memory device 74 are instructed by a vessel 76 to detach from the pedestal 72 and to rise to the ocean surface 78 to be picked up by the vessel 76.
However, this configuration is not very reliable as the mechanism maintaining the sensor 70 connected to the pedestal 72 may fail to release the sensor 70. In addition, the sensor 70 and pedestal 72 may not achieve their intended positions on the bottom of the ocean. Furthermore, the pedestals 72 are left behind and thereby contribute to both ocean pollution and a price increase, which are both undesirable effects.
Accordingly, it would be desirable to provide systems and methods that provide inexpensive and non-polluting nodes for reaching the seabed and recording seismic waves.